Interlace video cameras capture scenes as fields instead of frames. A field is half the vertical width (i.e. height) of a frame. A frame is composed of two fields. A frame of an interlaced video sequence is formed by combining alternate lines of a pair of fields.
Interlacing was originally designed for cathode ray tube (CRT) displays in which phosphor glows to illuminate the screen. One reason why interlacing works well on CRT displays is that human beings notice details flickering less than mass flickering. Nowadays, CRT displays are being replaced by displays having newer technologies, such as, for example, liquid crystal displays (LCD) and plasma displays. Both LCD and plasma displays use progressive scanning or frames as inputs, rather than fields.
To display a pair of two adjacent interlace fields as a progressive full frame, the fields must be combined together. However, the interlace fields are captured sequentially and, therefore, at slightly different times. Accordingly, vertical and diagonal structures or edges distort if the camera or the subject moves in-between the first and second fields being captured. This results in visually discernable artifacts (i.e. distortions) occurring when the two fields are combined together into a single frame. Exemplary artifacts, known as interlacing (or combing) artifacts, are shown in FIG. 1. FIG. 1(a) illustrates some signage, wherein interlacing artifacts can be clearly seen at the edges between the pale lettering of ‘FOST’ and the darker background behind. FIG. 1(b) illustrates a person pointing, wherein interlacing artifacts can be clearly seen at the edges between the pale post to the right of the person and the darker background behind. In addition to degrading image quality, the interlacing artifacts can reduce the coding efficiency of video encoders.
De-interlacing is a mechanism to remove or reduce as many interlacing artifacts as possible when two fields are combined into a frame. As interlacing artifacts are removed, visual quality and coding efficiency of the frame may be improved.